1. Field of the Invention
This invention relates to a process for reducing odor emission in aqueous polymer dispersions and in polymer powders.
2. Background Art
Polymer dispersions and polymer powders are used as binders in the building, coating and textile sectors. One problem of such dispersions and polymer powders is that they generally still contain volatile odor-intensive constituents such as mercaptans, which are used as chain transfer agents in polymerization; ammonia, which is used for neutralization; residual monomers; nonpolymerizable impurities of the monomers; volatile reaction products of the monomers that are formed under reaction conditions; and volatile degradation products of the polymers. The resulting odor is perceived as unpleasant not only by manufacturers but also by users/consumers, which is why there is a need for deodorized aqueous polymer dispersions and polymer powders.
In addition to the intrinsic odor of polymer dispersions, there are often odor sources which arise during processing. One example is in the production of fibers. Fibers are subjected to a high mechanical load in the course of processing. In order that they may be able to withstand this high mechanical stress, they are treated with pretreatment agents such as sizes, textile auxiliaries for dyeing and printing, and fiber finishes. Typical textile auxiliaries include dispersants (e.g. naphthalenesulfonates, fatty alcohol ethoxylates, polyacrylates), dyeing wetters, dyeing accelerants, dyes (e.g. indigo, sulfur dyes). Finishes include optical brighteners, agents for improving the wrinkling and creasing characteristics, antistats, hydrophobicizers, and flame retardants. All these additives are subjected to a high thermal load together with the polymers in the course of pressing operations. Decomposition reactions which take place lead to a multiplicity of chemical compounds which are responsible for an unpleasant odor.
It is known to deodorize polymer dispersions by physical or chemical aftertreatment. Physical processes, for example, include distillative processes, especially steam distillation, and also stripping with inert gases, as disclosed in EP-A 327006. The disadvantage of this process is that many dispersions are insufficiently stable for this kind of deodorization, so that coagulum formation occurs, which necessitates costly and inconvenient filtration before further use. The process has a further disadvantage, in that although it is able to reduce the fraction of volatile materials in the aqueous polymer dispersion, it does not resolve the issue of disposing of these materials.
It is also known to free polymer dispersions of odor-intensive monomers by chemical aftertreatment. For instance, DE-A 4419518 describes a free-radical postpolymerization using redox initiator systems to lower the level of residual monomers by chemical means. U.S. Pat. No. 4,529,753 describes a process whereby the level of residual monomers in aqueous polymer dispersions can be reduced by the action of particular free-radical redox initiator systems in a free-radical postpolymerization subsequent to the end of the main polymerization reaction. Such redox initiator systems comprise at least one oxidizing agent, at least one reducing agent and also one or more transition metal ions occurring in various valency states.
However, the above-recommended processes are disadvantageous in that, although they are in some instances able to lower the residual monomer content when used in odor-intensive polymer dispersions, they are not effectively able to control the unpleasant odor due to odor-intensive by-products, nonpolymerizable impurities of the monomers, volatile reaction products of the monomers, or volatile degradation products of the polymers. Nor are these measures able to effectively prevent the odor emission which is due to auxiliaries which are-used in the course of the processing of the polymer dispersions or polymer powders.
U.S. Pat. No. 4,906,462 discloses an odor-inhibiting composition which contains a phosphoric acid component, a copper compound and optionally a reducing agent. JP-A 62-204755 utilizes a combination of copper sulfate and ascorbic acid for deodorization in air-conditioning systems. GB-A 2340505 describes the use of imidazole compounds for odor minimization in natural gas and crude oil.
It is an object of the present invention to provide a process for effectively reducing the odor emission of polymer dispersions and polymer powders, in particular during their processing. These and other objects are achieved by admixing an aqueous polymer dispersion, post-polymerization, with a selected group of water soluble deodorizing compounds.